Speech level control for FM transmitter - has limiting clipper which prevents clipping of frequencies below one KHz by use of constant gain amplifier

摘要

A speech modulated FM transmitter has 6dB oer octave pre-emphasis over the range 300 to 3000Hz (1), followed by a clipper which limits peak frequency deviation to 4kHz. Severe distortion results from soeaking too loudly, particularly if clipping occurs on frequencies below 1000Hz. because the third harmonic is then in the audio pass band. Distortion is reduced by inserting a gain-controlled amplifier between the microphone and the pre-emphasis circuit. With increasing speech level, the gain remains constant until frequency components at 1000Hz reach the clipping level. Further increase in speech level reduces the gain progressively. This ensures that frequencies below 1000Hz are not cliooed (11).